1. Field of the Invention
The present invention is directed to a heat-sensitive recording material with a substrate on which a heat-sensitive recording layer containing color formers and color acceptors and a protective layer covering this heat-sensitive recording layer are arranged. The present invention is further directed to the use of the proposed heat-sensitive recording material as a ticket and particularly as a parking receipt ticket.
2. Description of the Related Art
Heat-sensitive recording materials have been known for many years and are steadily gaining in popularity. This may be explained by the fact that their use as tickets in particular offers great advantages to ticket suppliers. Because the color-forming components in the heat-sensitive recording process reside in the recording material itself, it is possible to employ large numbers of thermal printers which operate without toner or ink cartridges and whose function need no longer be monitored by persons at regular intervals. Accordingly, this innovative technology has had extensive success particularly in public transportation, busses and rail transportation, air travel, stadium and museum ticket kiosks, and parking receipt dispensers. Yet it is precisely in this very important area of application of parking receipts that numerous problems persist which have not so far been solved in their totality in a convincing manner.
Owing to the fact that parking receipt tickets can be exposed to direct sunlight at very high temperatures when placed behind the windshield, as is frequently required, it happens time and again that the ticket darkens heavily to the point of turning completely black. The print image produced by the thermal printer is then no longer legible, and the tickets become unusable already during their period of validity. Poor resistance of the heat-sensitive recording material used for the parking receipt ticket to grease and plasticizers further impairs the legibility of the printed information. However, parking receipt tickets very often come into contact precisely with these substances because grease often adheres to the fingers and hands of the user, and plasticizers are contained in sleeves into which the parking receipt tickets are inserted during their period of use. This effect is amplified when a paper web having recycled fibers is used as substrate because the recovered papers used for this purpose very often contain contaminants, particularly plasticizers, that can affect the resistance of thermally formed print images.
Another problem is the risk of jamming of ticket rolls after coming into contact with water. For example, when rolls or fan-folded stacks of parking receipt tickets of heat-sensitive recording material to be dispensed are loaded in automatic ticket dispensers in rainy weather, it may happen that these rolls or fan-folded stacks are dampened by raindrops and this moisture penetrates into the ticket rolls or fan-folded stacks. If the constituents particularly in the outer layers of the parking receipt tickets begin to dissolve, the individual layers within a roll or fan-folded stack of parking receipt tickets to be dispensed may stick together resulting in a total breakdown of the automatic ticket dispenser in question. Dust is another problem which arises when heat-sensitive recording material is formed in fan-folded stacks. In particular, heat-sensitive recording material with highly water-resistant protective layers is often very brittle so that the protective layers can flake off at the cut edges and folded edges. This causes dust and disrupts production.
European Patent EP-B-0 899 126 discloses a generic heat-sensitive recording material in which the diacetone-modified polyvinyl alcohol claimed herein as binder in the protective layer is suggested as a characterizing feature. However, an integral component of the known recording material which is indicated as having a good resistance to water, oil and plasticizers is also the use of dicarboxylic acid dihydrazide as an insolubilizer within the recording layer. Although 4,4′-dihydroxydiphenyl sulfone, as one of twenty color acceptors explicitly mentioned, is known from the prior publication along with other groups of alternative color acceptors mentioned for use in the heat-sensitive recording layer, obviousness of a combination of diacetone-modified polyvinyl alcohol as binder in the protective layer with 4,4′-dihydroxydiphenyl sulfone as color acceptor in the heat-sensitive recording layer can be entirely ruled out, especially since the stated objects to be met by the present invention such as improved resistance to heat, folding strength and flexural strength are not mentioned in the prior publication.
A generic heat-sensitive recording material is also disclosed in European Patent Application EP-A-1 900 541. The content of the disclosure of EP-A-1 900 541 does not go beyond the disclosure of the above-cited EP-B-0 899 126.